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Old Sep 03, 2012, 09:51 AM
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Great design problem. High power push-pull power for 80" seaplane

Single pylon mounted engines normally produce thrust far above the airframe's drag center producing numerous problems, pitch-power coupling perhaps the most serious. In short, if you stuff it it tries to dive, and every change in power or airspeed messes with trim. My proposed solution is to use two motors mounted in-line on a pylon, push-pull, props as small in diameter as possible with good efficiency and thrust, and shut down the aft motor in cruise flight for efficiency.
Dornier used this idea many times.
Issues I see:
1) 3-blade propellers to reduce clearance?
2) Aft engine running in propwash from forward engine, creating a neat design problem for aft prop- pitch? 3-blade folders are available for the rear engine.


Dr. Kiwi? Anyone? I'm really over my head here.
I could use some advice as to how to proceed. MotoCalc is not the answer.
The model will be a 2.2 meter, 1/8 scale(ish) version of an aircraft I built but never finished because of medical reasons.
Jim
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Old Sep 03, 2012, 01:11 PM
An itch?. Scratch build.
eflightray's Avatar
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What sort of weight and flight performance are you considering ? as that can make quite a difference on power requirements.
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Old Sep 03, 2012, 01:33 PM
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The aft motor is only used for take off? Sounds like something is wrong with the idea as it makes no sense to carry a motor that isn't needed in flight IMO I get it when it's a fully loaded B-36 but a non person or payload carrier? I also don't understand the how the CR function of the two props will correct the thrust problem, especially if one is shut down during flight. What I do know is the aft prop in a pullme pushme configuration is smaller in diameter by about 10%. to 20%.
Larger slower turning props are more efficient than small fast turning props in general.
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Old Sep 03, 2012, 03:01 PM
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Wow. I must have explained it badly.
If you think conventionally, all you say is true, but the reason this is a fun problem is that it's unconventional.
As I said, it's a five pound airplane (or is projected to be).
More detail:
1300 in. square, clark y thinned to 10%, v-tail and very clean for a seaplane.
All seaplanes have a big resistance hump - the transition to planing mode- that is absent in land planes, so they need a lot of power for about one minute, at takeoff.As well, a hull sliding through the water typically produces a lot more resistance than a rolling tire. I want to use two engines that, individually, would underpower the bird, but together will be enough to beat the hump and the drag handily, and then after climb out, in cruise mode,we shut down the rear one.
As I'm sure you know, an outrunner or inrunner brushless typically operates most efficiently at a particular current draw- often about 75-80% power. Size the motors right and you've got the best of both worlds- good takeoff power, and great power plant efficiency on the remaining motor when the second motor is shut down and prop is trailing.
In order to beat the nasty habits of displaced thrust line airframes, we need the smallest prop disc that can be efficient, mounted on a short pylon with minimum prop clearance above the fuselage..
There's a lot of similarity to EDF power here- in fact, the full size Dixie Skipper was designed with a ducted fan unit for power.

What's the best choice for a motor that will spin a small diameter prop , perhaps three bladed, (7"? 8" max diameter?) and produce the most thrust?
What will be the effect of the aft prop running in a slipstream that is already fast moving? Is contra-rotating the best? Will I need more pitch? More diameter? Less diameter?
What software can I find that will tell me what thrust is to be had with a particular motor and prop combination?
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Old Sep 03, 2012, 03:14 PM
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Originally Posted by eflightray View Post
What sort of weight and flight performance are you considering ? as that can make quite a difference on power requirements.
About 5 pounds, 2.2 meters span, 1.4 meters long, v-tail. 10% clark y. (?) With both motors running, it should be a hot number, but it should have a very efficient, sedate cruise with only the front motor running close to max. efficiency current drain.
See? A fun problem.
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Old Sep 03, 2012, 05:17 PM
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Salut le bourguignon !!

I don't think you'll find any calc dealing with push pull.

I don't believe three blade props are good for efficiency, neither small props as said by zeroback.

I understand what's the challenge here, large power for take-off and climb can limit the cruise efficiency, because of all the bad effects of very low throttle setting. An efficient setup should be just enough to fly, cruising, at full throttle.

A good answer I can see would be to have different applied voltage on the same motor (a small 4S for take-off and then a big 3S in cruise or something like that), but never take time to think about how it could be done !
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Old Sep 03, 2012, 05:18 PM
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Hastings, New Zealand
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Originally Posted by wazoo22 View Post
What's the best choice for a motor that will spin a small diameter prop , perhaps three bladed, (7"? 8" max diameter?) and produce the most thrust?
A small prop needs to spin fast to get high thrust, so you need a high Kv motor and/or high voltage battery. The motor should be designed to run efficiently at high rpm.

Quote:
What will be the effect of the aft prop running in a slipstream that is already fast moving?
This will reduce loading on the aft motor, but probably only have a significant effect at high speed.

Quote:
Is contra-rotating the best?
Yes. This will straighten out the airflow, increase load on the aft motor, and improve prop efficiency.

Quote:
Will I need more pitch? More diameter? Less diameter?
I would stick with the same diameter and pitch for both props. I tested a coaxial twin motor setup with 7x5 props, and found little difference in static loading between the front and rear. Thrust and power with both motors running was almost exactly double the amount produced by one motor alone.

Quote:
What software can I find that will tell me what thrust is to be had with a particular motor and prop combination?
I recommend Motocalc, as it calculates in-flight performance and does multi-motor setups. However, realize that the accuracy of any calc program is dependent on having good input data. Where possible you should use motor and prop 'constants' which have been derived from real-world test data.

Here is an example Motocalc estimation for your model. The first calc is for both motors running at full throttle, the second is cruising at 64% throttle with one motor shut down.

Motor: Lightflight Black 2814 1310Kv @14V; 1322rpm/V; 1.8A no-load; 0.062 Ohms.
Battery: Horeson 3300 45C (45C); 4 cells; 2950mAh @ 3.85V; 0.004 Ohms/cell.
Speed Control: Hobbywing Pentium 40; 2 controls (separate); 0.004 Ohms; High rate.
Drive System: APC 'E' 7x5 (ba); 2 motors (parallel); 7x5 (Pconst=1.5; Tconst=1) direct drive.
Airframe: Wazoo22 seaplane 2.2m; 1300sq.in; 80oz RTF; 8.9oz/sq.ft; Cd=0.044; Cl=0.37; Clopt=0.54; Clmax=1.01.
Stats: 159 W/lb in; 131 W/lb out; 15mph stall; 21mph opt @ 38% (40:19); 25mph level @ 43% (31:11); 1575ft/min @ 59.6; -188ft/min @ -5.9.
Conditions: Sea Level, 101.3kPa, 15C

AirSpd (mph) = 0.0 <--- static at WOT
Batt Amps = 55.3
Motor Amps = 55.3
Motor Volts = 14.4
Input (W) = 796.4
Loss (W) = 140.4
MGbOut (W) = 656.0
MotGb Ef(%) = 82.4
Prop RPM = 16777
Thrust (oz) = 83.0
PSpd (mph) = 79.4
Time (m:s) = 3:12

AirSpd (mph) = 55.0 <--- maximum level flight speed at WOT
Batt Amps = 48.8
Motor Amps = 48.8
Motor Volts = 14.5
Input (W) = 708.1
Loss (W) = 120.5
MGbOut (W) = 587.6
MotGb Ef(%) = 83.0
Prop RPM = 17200
Thrust (oz) = 48.0
PSpd (mph) = 26.4
Time (m:s) = 3:38

Drive System: APC 'E' 7x5 (ba); 7x5 (Pconst=1.5; Tconst=1) direct drive.
Airframe: Wazoo22 seaplane 2.2m; 1300sq.in; 80oz RTF; 8.9oz/sq.ft; Cd=0.044; Cl=0.37; Clopt=0.54; Clmax=1.01.
Stats: 26 W/lb in; 20 W/lb out; 15mph stall; 21mph opt @ 48% (42:08, 43C); 25mph level @ 54% (31:58, 48C); 169ft/min @ 5.3; -188ft/min @ -5.9.

AirSpd (mph) = 29.0 <--- cruising on one motor at 64% throttle, ~4mph above minimum level flight speed
Batt Amps = 8.52
Motor Amps = 13.3
Motor Volts = 9.59
Input (W) = 127.6
Loss (W) = 28.4
MGbOut (W) = 99.2
MotGb Ef(%) = 77.8
Prop RPM = 11400
Thrust (oz) = 13.5
PSpd (mph) = 25.0
Time (m:s) = 20:47
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Old Sep 03, 2012, 08:07 PM
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Smaller dia. more pitch on aft prop. That's the way it's done on stern drives with counter rotators on the same axial line. In some cases the aft prop has more blades than the fwd prop as well.
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Old Sep 04, 2012, 02:36 AM
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Originally Posted by yomgui View Post
Salut le bourguignon !!

I don't think you'll find any calc dealing with push pull.
Bonjor
I agree, but just a simple engine/prop/thrust calculator would give me a start. MotoCalc will sorta do it, but there you have all that other data mixed in, that clouds the picture..

I don't believe three blade props are good for efficiency, neither small props as said by zeroback.
That's the other challenge. Need to use small and probably three-blade to get thrust line down nearer drag center. Also produces a much cleaner frontal drag profile. Will this make up for the losses? Dunno.
I'll include a link to an interesting post, if I can find it, where an experimenter found improved thrust from a 3-blade. I'm aware of the traditional view of three or four blade efficiency loss, but we're stepping outside the traditional a bit here. I hope.

I understand what's the challenge here, large power for take-off and climb can limit the cruise efficiency, because of all the bad effects of very low throttle setting. An efficient setup should be just enough to fly, cruising, at full throttle.
Exactly! The benefits of such a setup could be large improvements in range and endurance, and perhaps a savings in weight because a smaller pack will suffice.

A good answer I can see would be to have different applied voltage on the same motor (a small 4S for take-off and then a big 3S in cruise or something like that), but never take time to think about how it could be done !
|Excellent Idea- moto calc can help there. Still leaves the thrust-line problem unsolved, and I suspect the weight of the second pack will exceed the weight of second motor. Still, I will work on that today, when I get time. Bon journe', mon ami. Merci.
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Old Sep 04, 2012, 03:37 AM
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Originally Posted by Bruce Abbott View Post
A small prop needs to spin fast to get high thrust, so you need a high Kv motor and/or high voltage battery. The motor should be designed to run efficiently at high rpm.

This will reduce loading on the aft motor, but probably only have a significant effect at high speed.

Yes. This will straighten out the airflow, increase load on the aft motor, and improve prop efficiency.

I would stick with the same diameter and pitch for both props. I tested a coaxial twin motor setup with 7x5 props, and found little difference in static loading between the front and rear. Thrust and power with both motors running was almost exactly double the amount produced by one motor alone.

I recommend Motocalc, as it calculates in-flight performance and does multi-motor setups. However, realize that the accuracy of any calc program is dependent on having good input data. Where possible you should use motor and prop 'constants' which have been derived from real-world test data.

Here is an example Motocalc estimation for your model. The first calc is for both motors running at full throttle, the second is cruising at 64% throttle with one motor shut down.

Motor: Lightflight Black 2814 1310Kv @14V; 1322rpm/V; 1.8A no-load; 0.062 Ohms.
Battery: Horeson 3300 45C (45C); 4 cells; 2950mAh @ 3.85V; 0.004 Ohms/cell.
Speed Control: Hobbywing Pentium 40; 2 controls (separate); 0.004 Ohms; High rate.
Drive System: APC 'E' 7x5 (ba); 2 motors (parallel); 7x5 (Pconst=1.5; Tconst=1) direct drive.
Airframe: Wazoo22 seaplane 2.2m; 1300sq.in; 80oz RTF; 8.9oz/sq.ft; Cd=0.044; Cl=0.37; Clopt=0.54; Clmax=1.01.
Stats: 159 W/lb in; 131 W/lb out; 15mph stall; 21mph opt @ 38% (40:19); 25mph level @ 43% (31:11); 1575ft/min @ 59.6; -188ft/min @ -5.9.
Conditions: Sea Level, 101.3kPa, 15C

AirSpd (mph) = 0.0 <--- static at WOT
Batt Amps = 55.3
Motor Amps = 55.3
Motor Volts = 14.4
Input (W) = 796.4
Loss (W) = 140.4
MGbOut (W) = 656.0
MotGb Ef(%) = 82.4
Prop RPM = 16777
Thrust (oz) = 83.0
PSpd (mph) = 79.4
Time (m:s) = 3:12

AirSpd (mph) = 55.0 <--- maximum level flight speed at WOT
Batt Amps = 48.8
Motor Amps = 48.8
Motor Volts = 14.5
Input (W) = 708.1
Loss (W) = 120.5
MGbOut (W) = 587.6
MotGb Ef(%) = 83.0
Prop RPM = 17200
Thrust (oz) = 48.0
PSpd (mph) = 26.4
Time (m:s) = 3:38

Drive System: APC 'E' 7x5 (ba); 7x5 (Pconst=1.5; Tconst=1) direct drive.
Airframe: Wazoo22 seaplane 2.2m; 1300sq.in; 80oz RTF; 8.9oz/sq.ft; Cd=0.044; Cl=0.37; Clopt=0.54; Clmax=1.01.
Stats: 26 W/lb in; 20 W/lb out; 15mph stall; 21mph opt @ 48% (42:08, 43C); 25mph level @ 54% (31:58, 48C); 169ft/min @ 5.3; -188ft/min @ -5.9.

AirSpd (mph) = 29.0 <--- cruising on one motor at 64% throttle, ~4mph above minimum level flight speed
Batt Amps = 8.52
Motor Amps = 13.3
Motor Volts = 9.59
Input (W) = 127.6
Loss (W) = 28.4
MGbOut (W) = 99.2
MotGb Ef(%) = 77.8
Prop RPM = 11400
Thrust (oz) = 13.5
PSpd (mph) = 25.0
Time (m:s) = 20:47
Bruce, I can't thank you enough. That's exactly what I'd hoped someone would be able to do. I use MotoCalc, but only found it recently, so I've not yet plumbed the depths there.
It seems that 100 watts is a bit much to maintain level flight, and I hope to run the front motor as close as possible to it's most efficient current drain, and 77% aint bad at all..
A great job. If I never did better than this, It would work fine.
Some questions, that might yield to your experience:

-Any ideas about a more efficient folding prop? I will need to use a folding prop on the rear, and it seems it would be good to keep them matched. I'll probably use one of these, because I can get them easily here in France.:
Aeronaut
Graupner cam carbon
How do they compare in efficiency to the APC? what's the (ba) mean? What's WOT?

-Inrunners have a smaller diameter, so less drag. Any ideas there?

-I notice the Hobbywing controller, two controls. Tell me more!

-How did you get MotoCalc to do twin engines? Is that the series-parallel check boxes? if so, why not series?
Did you use the wizard to find the Lightflight motor? I can't find any entry space to specify a twin there either.

Jim Miller
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Old Sep 04, 2012, 04:17 AM
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Quote:
Originally Posted by zeroback View Post
Smaller dia. more pitch on aft prop. That's the way it's done on stern drives with counter rotators on the same axial line. In some cases the aft prop has more blades than the fwd prop as well.
Wow. Weird.
Not contra-rotating, either.
I suspect it's not applicable because of the density difference and the fore and aft spacing difference, but very interesting.
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Old Sep 04, 2012, 04:48 AM
An itch?. Scratch build.
eflightray's Avatar
South Wales U.K.
Joined Mar 2003
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My Sunderland is 80" span, weighs 7Lb 2oz, so slightly smaller and heavier. WOT is about 600 watts.

It will take off water at less than full throttle, takes off from grass, (wet grass has more drag than dry), and again only need full throttle to get it to start moving on grass.

Initial trust is important to get a model moving. Small props give considerably less than larger diameter ones, they can make a lot more noise which may sound powerful, but at take-off they lose out.
The Sunderland works because it has four prop discs, that's a lot of area.

If you are going to use two motors, why not just mount them on the wing. You can use contra-rotating props if you think torque will be a problem, this also allows for differential throttle for great water steering. The lower thrust lines will reduce any thrust line affects that a high pylon may give. Many advantages, few disadvantages.

But the ultimate decisions are down to what you want it to look like.
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Old Sep 04, 2012, 05:58 AM
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Bruce Abbott's Avatar
Hastings, New Zealand
Joined Jan 2001
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Quote:
Originally Posted by wazoo22 View Post
-Any ideas about a more efficient folding prop? I will need to use a folding prop on the rear,
You don't have to use a folding prop. A small prop doesn't have a lot of drag when stationary.

Quote:
How do they compare in efficiency to the APC?
Folding props are generally less efficient than fixed props.

Quote:
what's the (ba) mean? What's WOT?
(ba) = tested by me ('ba' = bruce abbott).

WOT = Wide Open Throttle = full throttle.


Quote:
Inrunners have a smaller diameter, so less drag. Any ideas there?
The Mega 16/25/4 would be a good choice, though significantly more expensive than most equivalent outrunners. Using 28mm inrunners should reduce nacelle diameter by ~10mm.

Quote:
I notice the Hobbywing controller, two controls. Tell me more!
The Hobbywing Pentium/Flyfun 40A (AKA Turnigy Plush 40A, Exceed Volcano/Proton-40 etc.) is a reliable low cost 40 Amp ESC with a 3A switching BEC.

You need one controller per motor. Switching BECs don't like running in parallel, so you must remove the red wire from the signal cable of one controller (the other can be used to power your receiver and servos).

Quote:
How did you get MotoCalc to do twin engines? Is that the series-parallel check boxes?
Yes.

Quote:
if so, why not series?
You cannot run brushless motors in series.

Quote:
Did you use the wizard to find the Lightflight motor?
No. Motowizard is almost useless, so I don't use it. From experience I knew that a motor of that size and Kv would be about right, and I happen to have the Lightflight motor on my test stand right now (it's looking good so far!).
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Old Sep 04, 2012, 08:49 AM
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Bruce, I thought I could get around MotoCalc at least well enough to duplicate your results, but-- perhaps the twin engine thing is buggering up my attempts.
Best I could do was about 630 w. and 66 oz static with the same setup (?) And - what LiPo does 3.85 volts per cell? I thought the voltage might be my problem, but the manufacturer says 3.7 volts, like all the rest.
Would you be kind enough to run the problem again, and this time use 850 in. sq. for wing area? (brainfart). Otherwise the same basic data, and send me a screenshot, or some way to duplicate your setup. risico@wanadoo.fr
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Old Sep 04, 2012, 06:31 PM
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Bruce Abbott's Avatar
Hastings, New Zealand
Joined Jan 2001
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Quote:
Originally Posted by wazoo22 View Post
what LiPo does 3.85 volts per cell? I thought the voltage might be my problem, but the manufacturer says 3.7 volts, like all the rest.
3.85V is the typical resting voltage of a lipo cell at half charge. Unfortunately Motocalc uses 3.7V, which results in the calculated voltage being too low for modern high power batteries (even when impedance is reduced to zero). To fix this I change the chemistry to ' ', then increase impedance until the voltage matches my experience of real-world battery voltage under load.

I also turn motor heating effects OFF when calculating WOT performance, as Motocalc's temperature rise calculations assume continuous operation with relatively poor cooling. Usually I am more interested in the burst power available (for takeoff and in-flight aerobatics).

To simulate the effect of one motor being shut down I temporarily reduced the motor and ESC counts to 1, then increased the airframe weight to compensate.

Here are screen shots with the revised wing area. The main effect of lower wing area is increased speed.
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